For fast transmission of a large volume of information, there has been employed a system based on an optical wavelength multiplexing transmission technique. This optical wavelength multiplexing transmission system is for transmitting a wavelength-multiplexed light with different wavelengths through one optical fiber. In recent years, an optical wavelength multiplexing transmission system, which provides a transmission rate of approximately 2.4 Gbps×16 waves (where “G” represents 109 and “bit per second” signifies transmission rate per second), has been put to practical use.
Furthermore, on the design of a transmission line used for the optical wavelength multiplexing transmission, it is technically required to suppress the non-linear effects of an optical fiber. Still furthermore, in a case in which the aforesaid optical wavelength multiplexing transmission system is used as a linear repeating system, the key is maintaining the gain fattening in an optical band the optical amplifier puts to use. For this reason, as one example of meeting such a technical requirement, there is a method in which an optical amplifier is controlled through the use of ALC (Automatic Level Control).
An optical wavelength multiplexing transmission system will first be described with reference to FIGS. 18 to 20 and an ALC will then be described with reference to FIGS. 21 and 22. In the following description, an optical amplifier will sometimes be referred to as an “optical AMP” or simply as an “AMP”, and the contents thereof are the same.
FIG. 18 is a block diagram showing a transmission side WDM (Wavelength Division Multiplexing) terminal station. In FIG. 18, a transmission side WDM terminal station 100a is an optical wavelength multiplexing transmission apparatus designed to perform the multiplexing for a wavelength-multiplexed light and the demultiplexing thereof, and acts as a transmission terminal station. Moreover, this transmission side WDM terminal station 100a is made up of a multiplexing unit (MUX) 16a, an up main signal light amplifying unit 31, an OSC (Optical Supervisory Channel) light transmitting unit 113b, a control unit 113c and an optical output monitor 113a. 
The multiplexing unit 16a accomplishes optical coupling, and the up main signal light amplifying unit 31 compensates for a drop of an optical level at optical multiplexing and dispersion compensation, or in a transmission line or the like. Moreover, the optical output monitor 113a monitors an optical output level from a coupler (optical coupler) 60a in the up main signal light amplifying unit 31 to issue an output monitor value. This output monitor value is inputted to the control unit 113c to implement the output control of the up main signal light amplifying unit 31 and the information transfer to the OSC light transmitting unit 113b. 
In addition, the OSC light transmitting unit 113b is for wavelength-multiplexing a sub-signal light, functioning as a control signal, and a main signal light for the purpose of the supervisory control of a remote station (not shown). In this case, the sub-signal light is equally referred to simply as an “OSC light”. This OSC light transmitting unit 113b is for carrying out the switching control between the ALC control for automatically controlling the optical output level and the AGC control (Automatic Gain Control) for variably controlling the gain, and further for transmitting the number of transmission wavelengths forming the ALC setting information and the information such as a transmission rate to a linear repeater or reception side WDM terminal station lying on the downstream side. These ALC control and AGC control will be described later with reference to FIGS. 21 and 22.
In the following description, a main signal light signifies an optical wavelength-multiplexed signal to be linearly repeated between WDM terminal stations, while an OSC light (sub-signal light) represents a single wavelength light for supervisory control which does not pass through an optical amplifier and which is terminated at each of repeating sections. This OSC light does not affect the passing of a main signal light and does not pass through an optical amplifier, and is used as a supervisory control channel or pilot light. This “pass” signifies that it is in a communication state. Although a WDM terminal station is equally referred to as a “WDM terminal device”, in the following description it sometimes will be referred to simply as an “terminal station”.
Moreover, in FIGS. 18 to 20, the same reference numerals as those used above denote the same or equivalent functions, and the further description thereof will be omitted.
FIG. 19 is a block diagram showing a WDM linear repeater. In FIG. 19, a WDM linear repeater 100b is an optical wavelength multiplexing transmission apparatus, and operates as a linear repeater (linear repeating device). An OSC light transmitted from a former station is received by an OSC light receiving unit 113d located on the wavelength-multiplexed light input side and the reception level is inputted as a reception value to the control unit 113c. Moreover, a wavelength-multiplexed light outputted from the up main signal light amplifying unit 31 is monitored in the optical output monitor 113a and the output monitor value is inputted to the control unit 113c. In the control unit 113c, an OSC transmission value is calculated on the basis of these values and is outputted to the OSC light transmitting unit 113b to be transmitted to an adjacent station (next station). Thus, information is transmitted through the use of an OSC light different from a main signal light.
FIG. 20 is a block diagram showing a reception side WDM terminal station. In FIG. 20, a reception side WDM terminal station 100c is also constructed as an optical wavelength multiplexing transmission apparatus, and works as a transmission (reception) terminal station. A main signal light from a transmission line is amplified in the up main signal light amplifying unit 31 and, following this, in a demultiplexing unit (DMUX) 16b, a received wavelength-multiplexed light is demultiplexed into lights with optical wavelengths λ1 to λ8 and then outputted.
In addition, the OSC light is received by the OSC light receiving unit 113d and the OSC reception value is inputted to the control unit 113c. On the other hand, for the main signal light, an output light level is monitored in the optical output monitor 113a and inputted as an output monitor value to the control unit 113c. On the basis of these values, the control unit 113c outputs an output control signal to a variable attenuator 31b in the up main signal light amplifying unit 31.
As described above, the transmission side WDM terminal station 100a, the WDM linear repeater 100b and the reception side WDM terminal station 100c, shown in FIGS. 18 to 20, are connected through optical fiber transmission lines, and a wavelength-multiplexed light comprising a main signal light and an OSC light is transmitted from the transmission side WDM terminal station 100a through the downstream side WDM linear repeater 100b to the reception side WDM terminal station 100c. 
Furthermore, with respect to the output level control of an optical amplifier, a description will be given hereinbelow of ALC control and AGC control in a case in which the number of wavelengths to be multiplexed increases and decreases. The ALC control will first be described with reference to FIGS. 21(a) to 21(c) and the AGC control will then be described with reference to FIGS. 22(a) to 22(c).
FIGS. 21(a) to 21(c) are illustrations for explaining an ALC operation at an increase/decrease in number of wavelengths. As one example, a spectrum waveform shown in FIG. 21(a) has peak values L1 at two places on an optical wavelength axis (horizontal axis). In this state, when the number of wavelengths is decreased (contracted), one wavelength appears as shown in FIG. 21(b) and the peak value becomes higher than L1. On the other hand, if the number of wavelengths is increased (extended), three wavelengths appear as shown in FIG. 21(c) and the peak value becomes lower than L1.
FIGS. 22(a) to 22(c) are illustrations for explaining an AGC operation at an increase/decrease in number of wavelengths. In a state where peak values L2 exist as shown in FIG. 22(a), when the number of wavelengths is decreased, the peak values L2 remain intact as shown in FIG. 22(b). Even if the number of wavelengths is increased, the peak values L2 assume a constant value as shown in FIG. 22(c).
Meanwhile, a former-stage AGC amplifier 31a and a latter-stage AGC amplifier 31d are required to vary their output levels in accordance with the number of optical wavelengths to be multiplexed. If an increase/decrease in number of optical wavelengths takes place in a state where each of the optical amplifiers 31a and 31d conducts the ALC operation, each of the optical amplifiers 31a and 31d operates to maintain the optical output level to a constant value irrespective of the input level. Accordingly, in consequence, the output level per wavelength varies.
For this reason, at the increase/decrease in number of wavelengths, the control unit 113c switches the operation of each of the optical amplifiers 31a and 31d from the ALC control to the AGC control in a state where the ALC control level is fixed to a previous value, which makes no variation in optical output level for each optical wavelength even at the increase/decrease in number of wavelengths.
FIG. 23 is an illustration of a configuration of an optical wavelength multiplexing transmission system. In FIG. 23, in transmission lines, optical attenuators (optical PADs) 114 are provided, each of which attenuates the level of a wavelength-multiplexed light. The optical levels in the transmission lines are monitored by optical power meters 115 placed in the transmission side WDM terminal station 100a, the WDM linear repeater 100b and the reception side WDM terminal station 100c, respectively, thereby enabling the adjustment of the optical levels.
In the case of the optical wavelength multiplexing transmission shown in FIG. 23, the light to be transmitted is composed of multiple wavelengths and the non-linear effect due to the transmission optical fiber becomes noticeable; therefore, the optical power which can be sent to the transmission line has an upper limit. Moreover, due to the dynamic range determined by the NF (Noise Figure: noise characteristic) of an optical amplifier, the reception level of the optical amplifier is required to be severely adjusted up to several dBs.
For this reason, so far, for this optical input level adjustment, the optical level of a signal light has been measured by a level meter on the downstream side and the optical level adjustment has been made by the insertion of an optical attenuator (optical PAD) or variable attenuator (ATT) so that the optical level measured agrees with the input dynamic range, which requires a troublesome field adjustment work.
Accordingly, a supervisor is required to handle the troublesome field adjustment work resulting from a secular change of the optical PAD and others or environmental variation. Moreover, it is considered to eliminate the need for this manual work by automating the control and to implement the feedback control of the transmission level on the basis of the optical level (total optical level) of the total main signal light on the reception side.
However, the employment of this automation system becomes difficult because of the following circumstances (1) and (2) peculiar to a wavelength multiplexing transmission/linear repeating system, so effects are unobtainable.
(1) As a characteristic of an optical wavelength multiplexing transmission system, the total optical level varies in accordance with the number of wavelengths to be put to use. Moreover, in the case of varying due to the extension and contraction in the number of wavelengths or the shutdown (disconnection) of a wavelength-multiplexed light stemming from the troubles of lower-order (downstream) equipment, the total optical level varies in a short period of time. This provides a problem in that, if the control of the transmission optical level is made on the basis of the reception level of the total optical level, the disturbance of the optical level occurs due to the increase/decrease in number of wavelengths, the shutdown or the like, which leads to unstable transmission quality.
(2) Since a linear optical amplifying system issues an optical output only when a wavelength-multiplexed light is inputted thereto, it is required that the optical input is made through the use of a measuring instrument or the like at the adjustment in the field. This provides a problem in that, not until a wavelength-multiplexed light is actually inputted to an optical amplifier to make this optical amplifier initiate an optical output, the reception side receives a signal for the level adjustment.
In addition, Japanese Patent Laid-Open No. HEI 9-116504 discloses a method of measuring a property of an optical transmission line in which a wavelength-multiplexed signal light propagates, and an optical transmission line property measuring method in which a portion of light propagating on an optical transmission line is led to another optical transmission line for measuring optical power distributions of up and down signal lights with different wavelengths. However, this publication does not disclose the transmission/reception of control information using OSC light.
The present invention has been developed in consideration of these problems, and it is therefore a first object of the invention to, in an optical wavelength multiplexing transmission system which transmits a wavelength-multiplexed light while adjusting an optical level between optical amplifiers through the use of a main signal light and an OSC light, provide an optical wavelength multiplexing transmission apparatus and an optical output control method for an optical wavelength multiplexing transmission apparatus, capable of adjusting a transmission optical level through the use of the OSC light without delivering a main signal light by monitoring transmission output levels and receive input levels of the main signal light and the OSC light, thus enabling quick restoration from troubles.
A second object of the invention is to provide an optical wavelength multiplexing transmission apparatus and an optical output control method for an optical wavelength multiplexing transmission apparatus, capable of achieving stable calculation of an optical output level even when a change of the number of wavelengths of a main signal light to be multiplexed takes place.
A third object of the invention is to provide an optical wavelength multiplexing transmission apparatus and an optical output control method for an optical wavelength multiplexing transmission apparatus, capable of eliminating the need for a signal source for a receive optical level adjustment at the initial installation and eliminating the need for optical parts for the receive optical level adjustment by eliminating the troublesome adjustment for improving the reliability of the transmission lines, and capable of coping with a change with the passage of time on the transmission line loss after the field adjustment (level adjustment work in the field) or troubles, or a variation of the transmission line loss due to the moving without requiring the re-adjustment.